Today's rechargeable batteries, particularly those used to power portable electronic equipment such as personal computers, have a life-span limited to a few hours so that it is necessary to recharge them often. This is generally done by means of a power supply-transformer connected to the A-C mains.
In order not to damage the batteries and to prolong life-span as much as possible, it is necessary for charging to be performed with the maximum efficiency, without overloading the batteries, i.e. without supplying them a charge greater than the charge they can accept. The charge, or recharge, must therefore be interrupted at the proper time, when the battery is fully charged or at least close to termination of the charge.
It is known from U.S. Pat. No. 5,329,219 a method for establishing when to stop the charging of the battery, wherein the increase in temperature of the battery is detected in a given interval of time, during the recharge process, and this increase is compared with a predetermined reference value. By selecting a sufficiently small interval of time, the temperature increase is considered, with a sufficient degree of approximation, equivalent to the rate of change of temperature with respect to time.
When the temperature increase in the given interval of time, i.e. the rate of change of temperature, exceeds the reference value, it is considered that the battery has reached, or is close to, its maximum allowable charge level and as a result the current supplied to the battery is switched to a lower current, known as trickle current.
It is also known to any person skilled in the art a method for interrupting wholly the supply of current to the battery, at the exceeding of the reference value.
However, these methods of establishing if the battery has reached its maximum permissible charge level are effective and dependable only when the charging current supplied to the battery by the power supply is substantially constant and sufficiently high, as the reference value is set for a given charging current only.
In practice however, it often happens that the charging current supplied to the battery is lower than the set value and, in an extreme situation, is of a value close to zero. This may happen, for example, when the battery is being charged and the electronic equipment using it, a portable personal computer for example, is in operation at the same time. This is due to the fact that the electronic equipment, to power its operating circuits, normally absorbs a large amount of the current supplied by the power supply which is also powering the battery. As the current supplied by the power supply remains substantially constant in time and is not normally susceptible to increases, the current available is obviously shared, with consequent reduction and variability of the current powering the battery.
With less current reaching the battery, temperature of the latter rises more slowly so that its rate of change in time also remains at low levels, which could be practically negligible and, in any case, permanently under the reference value or threshold, if this value were chosen in relation to a high charging current, and it would therefore be difficult to detect when the battery is charged. As a result, the battery would continue to be charged beyond its capacity and could possibly, in an extreme case, explode.